This invention relates to a method of optimizing the supply of energy to an electrostatic precipitator and of avoiding a reverse corona discharge in such precipitator, which is supplied with energy in the form of d.c. voltage pulses or of a d.c. voltage and superposed pulses.
In a known method of that kind, disclosed in Published German application No. 30 72 172, an electrostatic precipitator is energized with a variable d.c. voltage and superposed pulses in such a manner that the d.c. voltage and/or one of the parameters of the pulses are automatically varied in an iterative manner so that the sum of the electric energy supplied to the precipitator in the forn of the d.c. voltage and the pulses will be minimized whereas the dust content of the pure gas will be maintained at a predetermined mean value.
Published German Application No. 31 14 009 discloses an electrostatic precipitator in which dust is collected, in that a high d.c. voltage controlled by a thyristor is applied between the collecting electrodes and the corona electrodes. Besides, a control circuit is provided for an intermittent activation of the thyristors in such a manner that the repetition period and/or the pulse width of the high d.c. voltage can be manually or automatically adjusted. By means of that control circuit it is intended to improve the dust collection rate of the electrostatic precipitator particularly if the dust has a high resistivity in a range from 10.sup.11 to 10.sup.13 ohm-cm, in which an electrostatic precipitator normally does not operate satisfactorily owing to reverse corona discharges.
In the known control circuit the thyristor is controlled in such a manner that the high d.c. voltage is applied during a first interval of time T.sub.1 of, e.g., 0.001 to 1 second and its application is interrupted for a second interval of time T.sub.2 of, e.g., 0.01 to 1 second. The ratio of T.sub.1 to (T.sub.1 +T.sub.2), i.e., of the pulsing time to the total of the pulsing and nonpulsing times, in each control cycle can be described as the k value. The entire method is described as a "control by means of semipulses".
A special object of the known method is to avoid reverse corona discharges, which are reflected in the current-voltage characteristics by a comparatively very steep rise of the current in response to an only slight increase of voltage. Such characteristics cause the electrostatic precipitator to have a high energy consumption whereas its dust collection rate is only low. Because the occurrence of reverse corona discharges is somewhat delayed relative to the rise of voltage or current caused by the conventional control, the occurrence of reverse corona discharges can be substantially suppressed by the method employing semipulses and an economical operation of the electrostatic precipitator can thus be achieved.
In accordance with the proposals discussed hereinbefore and other known proposals, the measures adopted for an improvement of the dust collection rate, on the one hand, and for an optimizing of the energy consumption, on the other hand, are not adequately distinguished from each other. In many cases reference is made to the optimizing of the dust collection rate although that requirement will generally make sense only if the energy consumption involved has no economic significance.
From the aspects of engineering and economy, the consideration must be based on a predetermined dust content of the pure gas and the electrostatic precipitator must be operated at a dust collection rate which will ensure that that predetermined dust content of the pure gas will be achieved with the lowest possible energy consumption. But even that requirement is not adequately specific for the operation of an electrostatic precipitator which is powered with pulsed energy. The collection of a dust particle from a gas stream will require in the first place that as many charge carriers as possible are introduced into the gas stream so that the dust particles will be effectively ionized, and in the second place that the dust particle which has been optimally ionized will travel an adequate distance transversely to the direction of flow to reach a collecting electrode during the time which is available for the passage of said ionized particle through the precipitator. Said two partial functions will be accomplished by the electrostatic precipitator if the corona electrodes and collecting electrodes are properly disposed and if the precipitator is strictly electrically controlled by pulses. The dust particles are ionized during the pulses and are collected on the collecting electrode during the non-pulsing times. For this reason the general requirement to minimize the energy consumption requires, in more specific language, that the energy supplied to the electrostatic precipitator during the pulsing times should be maximized whereas the total energy consumption should be minimized.
It is an object of the invention to optimize the energy supply in the sense stated above and to avoid reverse corona discharges in a method of the kind described first hereinbefore.